An iterative noncoherent detection method of coded and symbol interleaved differential space-time modulation (DSTM) is proposed in flat Rayleigh fading channels. Like differential phase shift keying (DPSK), DSTM can be considered as a recursive convolutional code. A soft-output a posteriori probability (APP) decoder of DSTM is proposed in flat Rayleigh fading channels. In the decoder, multiple-symbol differential detection (MSDD) is used to calculate the branch metric. Channel autocorrelations are considered in MSDD to mitigate the effects of channel variation, in the Rayleigh fading channels. Simulation results demonstrate that the proposed detection method has good performance in fast Rayleigh fading channels and the performance can be enhanced by increasing both the iterative times and the observation intervals.